Manual Técnico - Chiller YORK® Absorção YPC ... - Johnson Controls

FORM 155.17-EG4
Available Draft (Da) – The Available Draft is the draft
required at the outlet exhaust flange of the YORK High
Temperature Generator.
Pressure Drop (dP) – Frictional losses in the chimney
system which act against theoretical draft. The chimney
draft needed to overcome chimney frictional losses
is described as follows:
Dt = dP + Da
Proper chimney design balances the theoretical draft
(Dt) against the pressure drop (dP) of the chimney system
in order to provide the required available
pressure(Da) at the outlet of the chiller-heater.
Proper chimney design must provide the required (Da)
under all operating conditions. Because the difference
between summer and winter ambient conditions can result
in Dt variations of 50% and greater, some method of
draft control is usually required in order to maintain Da.
Chimney Application – The YORK direct-fired chillerheater
is equipped with a forced draft burner capable of
firing on a variety of fuels, including natural gas and/or
No. 2 oil and/or propane. As such, the unit will require a
properly designed chimney system to control draft and
discharge flue gases from the unit to the atmosphere.
The combustion chamber of the chiller-heater is engineered
to product a positive gauge pressure of 0.05 to
0.15 in. of water (1.27 - 3.81 kg/m 2 ) at the outlet of the
first stage generator with an exhaust temperature of
400°F+/-50°F (204°C +/- 28°C). As such the chimney
design must provide a method for maintaining the pressure
at all ambient conditions. Because YORK
chiller-heaters operate at “high fire” throughout the summer
months, it is important to design the chimney system
for summer ambient design conditions to avoid
undersizing. It is recommended that the chimney itself
should be designed for a Da of 0 (zero) in. (0 mm) of
water column. This will prevent the chimney from becoming
pressurized at any point along the flue gas path.
Caution – If the stack/chimney pressure is ever
above 0 (zero) in. of water column there
is a chance of flue gas being leaked
into the equipment room.
There are two commonly used ways to maintain the
pressure at the outlet of the chiller-heater. Either manual
or automatic/motorized draft control can be used. All
YORK direct-fired chiller-heaters will come standard with
a manual draft control damper. This damper can be modified
for motorized operation either in the factory (if ordered)
or in the field if site conditions require.
Manual Draft Control – Manual draft control is suitable
for applications where each gas fired appliance will have
its own dedicated chimney and draft control system. Fig.
7 depicts a simple yet effective means of controlling draft.
YORK INTERNATIONAL
With this system, a (field supplied) barometric draft regulator
is used in series with the factory supplied manual
backdraft damper. With maximum economy employed
in the chimney design, Dt would exactly equal dP + Da
during the summer design ambient conditions with the
barometric regulator closed. In reality, some degree of
conservatism should exist in the design, causing the
barometric regulator to be open slightly even during
summer design conditions. As ambient temperatures
drop, Dt would increase, if not for the barometric draft
regulator. With the regulator in place, mechanical room
air is introduced into the chimney system in response
to the impressed draft, thus stabilizing the gauge pressure
just upstream of the barometric regulator. Most
barometric regulators can maintain -0.06 in. water column
gauge pressure when properly sized for a particular
application.
With the gauge pressure stabilized upstream of the barometric
regulator, the factory supplied manual backdraft
damper can be adjusted to a fixed position which will
provide the pressure drop to yield the required positive
pressure at the exhaust flange of the chiller-heater.
Motorized Draft Control – Motorized draft control is suitable
for applications where multiple gas fired appliances
will be ducted into one common chimney system. In this
case each unit will require its own draft control system.
(Motorized draft control may be used for one chiller-heater/one
chimney applications as well if it is desired
over manual control.) Fig. 8 depicts a sequential
draft control system which incorporates a motorized
damper whose position is automatically adjusted as a
function of available draft at the outlet of the chiller. The
YORK supplied backdraft damper at the outlet of the
chiller can be modified (in the factory if ordered, or in
the field) to mount the motor driver. The motor is controlled
from a draft control panel which senses the pressure
at the outlet of the chiller-heater. The draft control
panel is available from YORK to ship with the chiller.
The panel is wired to the burner panel and damper motor
in the field, and the pressure is sensed through a small
line field connected to the outlet of the chiller-heater.
Special Problems and Maintenance – Factors causing
draft variations during normal operation include: wind
and weather factors, inadequate chimney construction
or system installation, location of installation, or inadequate
system maintenance.
Wind and weather – Windy conditions will tend to increase
the draft in the chimney as the wind helps to
remove the combustion products leaving the chimney
at a much faster rate. Down draft may occur causing a
temporary positive pressure in the chimney system. The
stack should be designed to prevent not only wind, but
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